Abstract: The invention relates to a method for checking the operability of an exhaust gas after-treatment system (12) of an internal combustion engine by evaluating signals (S—22, S—26) of a first exhaust gas sensor (22) and a second exhaust gas sensor (S—26), between which a catalyst 24 is disposed. The method is characterized in that the air ratio L of an exhaust gas atmosphere flowing through the exhaust gas after-treatment system (12) is reduced from a first lambda value 11>1 to a third lambda value L3 with L2>L3 via a second lambda value L2 with L1>L2>1, the value of a time duration (dt—26; dt—22) is detected, which is between the times (t5, t6, 7t7, t8) at which the signals (S—26, S—22) of the two exhaust gas sensors (22, 26) display the second value L2, the value of the time duration (dt—26; dt—22) is compared to a threshold value (dt_max), and an analysis of the operability of the catalyst (24) is not carried out if the value of the time duration (dt—26; dt—22) is greater than the threshold value.